1. Field of the Invention
The present invention relates to a protection switch of the circuit breaker kind having in an isolating case a thermal bimetallic strip tripping device, and it relates more particularly to a device for adjusting the value of the thermal current which causes tripping of the circuit breaker should an electric overload occur, that is to say on the passage of a current exceeding the nominal value of the circuit breaker.
2. Description of the Prior Art
From the patent application FR 2 585 180 a mechanism is already known for adjusting, not only in the factory but also by the user, the thermal tripping current of a circuit breaker through the action of a bimetallic strip. This adjustment mechanism includes:
a wheel accessible from outside the circuit breaker case and having inner toothing, the wheel carrying graduations which correspond to direct or relative current values and which may be brought, by rotating the wheel, opposite an index carried by the case;
an inner flange centered on the wheel, adapted to be driven in rotation, having a member meshing with the teeth of the wheel, and formed with a groove of excentric shape with respect to the axis of rotation of the wheel;
a piece forming a tumbler mounted in the case and rotatable about an axis perpendicular to the axis of rotation of the wheel through a drive member engaged in the groove of the flange, the axis of rotation of the tumbler being fixed with respect to the case.
In the above mentioned application, the tumbler has a pivoting hook with axis parallel to the axis of rotation of the tumbler, which hook has a head cooperating with a nose piece provided at the end of an auxiliary tripping lever and a tail end spaced apart from the bimetallic strip by a given distance depending on the chosen value of the thermal current. Should an electric overload occur, the bimetallic strip urges the hook and forces it to pivot so as to free it from the nose piece of the tripping lever, which pivots so as to cause tripping of the circuit breaker.
For adjusting the thermal tripping current of the circuit breaker in the factory using the above described mechanism, the wheel is first of all locked in a position indexing its corresponding graduation to the minimum value of the thermal current, and the circuit breaker is placed in the set position in which the head of the hook is engaged with the nose piece of the tripping lever. From these initial positions, the flange only is rotated in a suitable direction so as to cause rotation of the tumbler-hook assembly, through the drive member engaged in the groove of the flange, until the tail end of the hook comes into contact with the bimetallic strip. Then, rotation of the flange in the same direction as before causes the hook to pivot about its bearing point on the bimetallic strip, so that the head of the hook is freed from the nose piece of the lever, thus causing tripping of the circuit breaker. The position of the flange which has just caused tripping of the circuit breaker defines a so called zero reference position from which the flange is then rotated in the opposite direction through a predetermined angle so as to move the tail end of the hook over a predefined distance corresponding to the travel of the bimetallic strip for the graduation corresponding to the minimum value of the thermal current initially indexed with the wheel locked. The factory adjustment is then finished and the wheel is then released and interlocked with the flange through the engagement member.
However, the use of this adjustment procedure has a major drawback. In fact, since the pivoting hook is carried by the adjustment tumbler, this latter may occupy initially with respect to the nose piece of the tripping lever a position which is different from one circuit breaker to another in the set position; under these conditions, during adjustment in the factory, the force exerted by the bimetallic strip on the hook for freeing this latter from the nose piece of the lever so as to cause tripping of the circuit breaker, varies depending on the initial position taken by the hook, which inevitably leads to a certain inaccuracy in the final adjustment.